This invention relates to controls for wound rotor AC induction motors, and particularly to such controls for bridge, trolley and hoist applications in crane drives. Such motors are generally provided with three phase rotor windings connected to an external three phase resistive load, the load resistor network including groups of shorting switches or contactors activatable during motor acceleration to progressively short out more of the load in steps as rotor speed increases, and vice versa for deceleration. In the past, this has generally been accomplished by the use of current relays and timers. These devices, however, are subject to maintenance and operator abuse problems, well known to those in the field, which can lead to reduced motor life. For example, the motors can be started under heavy loads or run up against stops or with brakes applied in ways that cause motor current to reach levels much greater than maximum rated current for significant periods of time. Such problems would be minimized if the shorting switches were activated strictly in response to a variable indicating rotor speed.
The prior art has some suggestions along these lines. U.S. Pat. No. 1,118,296 to Lutz, issued Nov. 24, 1914, shows a wound rotor motor control in which an external frequency changer driven by the rotor provides an AC feedback signal having a frequency which varies inversely with rotor rotational speed from line frequency at zero speed to zero frequency at synchronous speed. This is applied to electromagnetically actuated contactors having different magnetic characteristics designed to actuate the contactors at the predetermined rotor speeds. This apparatus, however, is cumbersome, expensive and inflexible. U.S. Pat. No. 2,872,633 to Schurr, issued Feb. 3, 1959, shows a wound rotor motor control in which rotor electrical frequency is used as an indication of rotor speed. This frequency itself varies from line frequency at zero rotor speed to zero frequency at synchronous speed, thus deleting the need for the frequency changer of Lutz. However, the signal is applied to resonant circuits to actuate the contactors; and this is an expensive and inflexible approach.
In addition, the prior art shows no awareness of the similarity in appearance between a loss of the signal voltage indicating rotor speed and a signal voltage of zero frequency indicating synchronous speed. Such a loss of rotor signal, for whatever reason, may cause the prior art controls to short out the entire load resistance, regardless of the actual rotor speed, and therefore deviate from desired control behavior.